The present invention generally relates to an internal combustion engine (hereinbelow, referred to as an "engine") and more particularly, to an intake arrangement for the engine, in which a combustion chamber of each of the cylinders is provided with a plurality of, for example, two intake ports each having an intake valve for opening or closing each of the intake ports such that at least one of the intake ports is selectively used in accordance with a load applied to the engine.
Conventionally, in intake arrangements for reciprocating engines equipped with a plurality of cylinders each having a combustion chamber and a cylinder head, it is known that two intake ports having a substantially identical bore area are provided on the combustion chamber so as to secure a large intake area and an intake passage formed in the cylinder head is connected to the intake ports at a small angle with an axis of the combustion chamber so as to cause intake gases to flow into the combustion chamber substantially along the axis of the combustion chamber such that the engines can yield high outputs by maximizing charging efficiencies of the engines. The above-known two-port type intake arrangements are advantageous in that it becomes possible to obtain high outputs from the engines under high load engine operating conditions, but have such inconveniences that low flow velocity of the intake gases, inferior combustibility of an air-fuel mixture, uneconomical fuel consumption and an increase of harmful compositions in the exhaust gas are incurred under low load engine operating conditions.
In order to eliminate the above described drawbacks, Japanese Patent Laid-Open Publication No. 44419/1981 (Tokkaisho 56-44419), for example, discloses an intake arrangement for an engine, in which a first branch intake passage (for low loads) and a second branch intake passage (for high loads) provided with a shutter valve are led from a main intake passage so as to be, respectively, connected to two intake ports formed on the cylinder head and the shutter valve of the second branch intake passage is closed under the low load engine operating conditions such that intake is performed by using only the first branch intake passage. More specifically, the main intake passage is provided with a throttle valve and the throttle valve is operatively associated with the shutter valve such that the shutter valve is opened upon opening of the throttle valve beyond a predetermined opening degree. Namely, under the low load engine operating conditions, since the second branch intake passage is closed by the shutter valve, the intake gases are supplied at a relatively high flow velocity through only the first branch intake passage. Meanwhile, under the high load engine operating conditions, since the intake gases are supplied through both the first and second branch intake passages, a large charging amount of the intake gases can be secured.
However, since this prior art intake arrangement is originally designed to secure a sufficiently large amount of the intake gases under the high load engine operating conditions, each of the first and second branch intake passages has a rather large cross-sectional area. Accordingly, even if only the first branch intake passage is used under the low load engine operating conditions, it is impossible to increase a flow velocity of the intake gases sufficiently under such extremely low load engine operating conditions as idling, so that it becomes impossible to effectively produce swirling motions of an air-fuel mixture in the combustion chamber, which swirling motions are indispensable for improvement of combustibility of the air-fuel mixture. It may be imagined that such a problem can be solved by simply reducing the cross-sectional area of the first branch intake passage. However, since the first and second branch intake passages are, respectively, connected to the intake ports substantially along the axis of the combustion chamber so as to increase an output of the engine as described above, effective swirling motions of the air-fuel mixture along a circumferential direction of the combustion chamber cannot be produced even if a flow velocity of the air-fuel mixture in the combustion chamber is increased by increasing the flow velocity of the intake gases, thereby resulting in rapid reduction of the flow velocity of the air-fuel mixture in a compression stroke. Meanwhile, in the case where the cross-sectional area of the first branch intake passage is reduced extremely so as to increase the flow velocity of the intake gases as high as possible, a load range usable in the engine is restricted accordingly, so that it becomes necessary to supply the intake gases also from the second branch intake passage by opening the shutter valve even under relatively low load engine operating conditions. At this time, since the two intake ports are formed symmetrically with a horizontal center line of the combustion chamber, the intake gases drawn from the first branch intake passage are caused to collide with those drawn from the second branch intake passage, so that swirling motions of the air-fuel mixture are impaired or almost extinguished and thus, excellent combustibility of the air-fuel mixture based on its vigorous swirling motions cannot be obtained.
In order to solve the above described problem, in Japanese Patent Application No. 176776/1983 (Tokugansho 58-176776) filed simultaneously with Japanese Patent Application No. 176777/1983 (Tokugansho 58-176777) on which the present application is based for priority, the assignee assigned by the present inventors proposed an intake arrangement for an engine having a combustion chamber, a cylinder head and a cylinder block, in which a plurality of intake ports opening into the combustion chamber are opened or closed by respective intake valves and are connected to an intake passage provided with a shutter valve. It is so arranged that the shutter valve is closed under low load engine operating conditions and is opened under high load engine operating conditions such that a cross-sectional area of the intake passage is reduced or increased in accordance with engine operating conditions. Meanwhile, an auxiliary intake passage having a cross-sectional area smaller than that of the intake passage is branched from a bottom portion of the intake passage so as to be connected to one of the intake ports, with the bottom portion being disposed upstream of the shutter valve.
Namely, in the intake arrangement of the present invention as referred to above, intake gases are exclusively supplied from the auxiliary intake passage into the combustion chamber through the intake port connected to the auxiliary intake passage at a high flow velocity under the low load engine operating conditions in which the shutter valve is closed. In this case, since the auxiliary intake passage is formed on the bottom portion of the intake passage, the auxiliary intake passage extends at a small angle with a mating face between the cylinder head and the cylinder block, so that the intake gases drawn into the combustion chamber are swirled in a circumferential direction of the combustion chamber. Furthermore, under the high load engine operating conditions in which the shutter valve is opened, the intake gases are drawn from a plurality of the intake ports into the combustion chamber at a high charging efficiency, so that the engine can positively yield a high output.
Meanwhile, an intake arrangement apparently similar to that of Japanese patent application No. 176776/1983 referred to above is proposed in Japanese Patent Laid-Open Publication No. 25511/1980 (Tokkaisho 55-25511) which discloses an internal combustion engine equipped with a plurality of cylinders each having a combustion chamber formed with two intake ports such that the two intake ports are, respectively, connected to two branch intake passages of a main intake passage. In this known intake arrangement, the two branch intake passages are formed by a partition wall provided adjacent to the combustion chamber. Furthermore, two auxiliary intake passages for obliquely injecting a portion of the intake gases into the combustion chamber are, respectively, provided in the branch intake passages such that outlet ports of the auxiliary intake passages are, respectively, oriented towards opposite portions on an inner face of the cylinder, which opposite portions are substantially symmetrical with the partition wall. Each of the auxiliary intake passages has a cross-sectional area smaller than that of each of the branch intake passages so as to open into each of the intake ports such that the intake gases are drawn from the small auxiliary intake passages into the combustion chamber at a high velocity under low load engine operating conditions. In this known intake arrangement, it is so arranged that the high-speed intake gases drawn from the two outlet ports of the auxiliary intake passages under the low load engine operating conditions produces, through interference therebetween in the vicinity of a spark plug provided at a substantially central portion of the combustion chamber, intense turbulent flow of an air-fuel mixture in the vicinity of the spark plug after having collided with the inner face of the cylinder so as to effect stable combustion of the air-fuel mixture by scavenging around the spark plug effectively and increasing its flame propagation velocity. Consequently, this known intake arrangement is entirely different, in objects, from the intake arrangement of the above Japanese patent application No. 176776/1983 intended to produce vigorous swirling motions of the air-fuel mixture in the circumferential direction of the combustion chamber. Moreover, in this known intake arrangement, although the auxiliary intake passages produce the expected effects under extremely low load engine operating conditions such as idling, it becomes impossible to secure a sufficiently large amount of the intake gases only through the auxiliary intake passages even under relatively low load engine operating conditions, so that it becomes necessary to open the branch intake passages communicating with the intake ports, respectively. Thus, when the branch intake passages are opened even slightly, flow velocity of the intake gases drawn from the auxiliary intake passages drops sharply, thereby imparing the turbulent flow of the air-fuel mixture in the combustion chamber. Accordingly, in this known intake arrangement, it becomes impossible to effect stable combustion of the air-fuel mixture under engine operating conditions ranging from the relatively low load engine operating conditions slightly higher than those of idling to medium load engine operating conditions.